Furnace for the combustion of waste liquors or other solutions or suspensions containing combustible organic substance and various combinations with alkali

ABSTRACT

Disclosed is a furnace for the combustion of waste liquors from chemical pulping processes or similar solutions containing organic substances and alkali. The furnace is cylindrical and is characterized by an upper annular collecting channel formed between the upper portion of the annular furnace walls and an outer circumferential wall having a vault thereover for forming the top wall of the furnace. The annular channel collects gases formed during combustion which rise to the top of the furnace and removes the gases from the furnace through a discharge passageway formed in the outer circumferential wall. The upper portion of the annular furnace wall is tapered so as to form a relatively larger distance between the furnace wall and vault on a side of the channel opposite the discharge passageway and a relatively smaller distance between the furnace wall and vault on a side of the channel near the discharge passageway.

Unite States Hasselberg aen 1 r [75] Inventor: Sven Rudolf GunnarHasselberg,

Broby, Sweden [73] Assignee: Broby Industri er AB, Broby,

Sweden [22] Filed: Feb. 8, 1973 [21] Appl. No.: 330,508

Related US. Application Data [62] Division of Ser. No. 60,532, Aug. 3,1970,

abandoned. l

[30] Foreign Application Priority Data Mullen 23/277 R X Dec. 3, 19742,261,995 1 H1941 Greenawalt 23/277 R X 2,806,768 9/1957 Bender et a123/277 R X FOREIGN PATENTS OR APPLICATIONS 162,004 0/1958 Sweden 227,1540/1969 Sweden Primary Examiner-S. Leon Bashore' Assistant ExaminerAlfredDAndrea, Jr.

[5 7 ABSTRACT Disclosed is a furnace for the combustion of waste liquorsfrom chemical pulping processes or similar solutions containing organicsubstances and alkali. The furnace is cylindrical and is characterizedby an upper annular collecting channel formed between the upper portionof the annular furnace walls and an outer circumferential wall having avault thereover for forming the top wall of the furnace. The annularchannel collects gases formed during combustion which rise to the top ofthe furnace and removes the gases from the furnace through a dischargepassageway formed in the outer circumferential wall. The upper portionof the annular furnace wall is tapered so as to form a relatively largerdistance between the furnace wall and vault on a side of the channelopposite the discharge passageway and a relatively smaller distancebetween the furnace wall and vault on a side of the channel near thedischarge passageway.

4 Claims, 3 Drawing Figures PATENTEB BEE 3 4 susa'ma FURNACE FOR THECOMBUSTION'OF WASTE LIQUORS OR OTHER SOLUTIONS OR SUSPENSIONS CONTAININGCOMBUSTIBLE ORGANIC SUBSTANCE AND VARIOUS COMBINATIONS WITH ALKAIJI Thisis a (x) division, of application Serial No. 60,532, filed Aug. 3, 1970,now abandoned.

The present invention relates primarily to a method for the combustionof waste liquors or other solutions or suspensions containingcombustible organic substance and various combinations with alkali, inwhich method the liquor evaporated to suitable concentration is burnedcontinuously in a furnace into which the liquor is sprayed and in whichthe alkali included in the liquor is converted into a soda smelt whichis discharged from the bottom of the furnace.

The term liquor as used herein is generally to be understood as wasteliquors from chemical pulping processes of cellulosic raw material,containing various combinations with alkali in or without the presenceof sulphur-containing substances. The liquors may be e.g., waste liquorsfrom e.g. the preparation of sulphite pulp, neutral sulphite pulp orso-called NSSC pulp, soda pulp or sulphate pulp. Also other types ofwaste liquors containing dissolved or suspended organic combustiblematerial and various combinations with alkali may come in question, e.g.solutions of sodium formate, etc.

It is known since long in the cellulose industry to burn evaporatedwaste liquors in so-called soda pans for generating steam for heat andpower production and for converting sodium compounds included in theliquor into a soda smelt which depending on the cooking method maycontain sodium carbonate (Na Co lor imixture of varying amounts ofsodium carbonate and sodium sulphide (Na s).

Plants of this type are very expensive. The combustion chamber in thesoda pan is, for instance, provided with cooled walls, and material anddesign must be chosen with great care in order to avoid e.g. corrosiveattacks. The cost for small cellulose mills becomes so high that it maybe doubtful whether they are economically profitable. Also, in manycases the content of combustible material in the liquor may be too smallto make steam generation economical.

This is the case in, for instance, the preparation of semichemical orother high-yield pulps in the yield range of 70 to 90 percent ofincluded lignocellulosecontaining material.

The object of the present invention is to make an essential contributionto an economical solution of the problem of neutralizing waste liquorsand recovering chemicals included therein also in smalll mills andparticularly in semichemical pulp mills.

The invention will now be described in more detail with reference to theaccompanying drawings in which:

section and a vertical longitudinal axis, a cyclone evaporator 3connected to the outlet 2 of the furnace for direct evaporation ofliquor, a suction fan 4 located after the evaporator 3 for sucking thegases from the evaporator 3 through a pipe 5 to an outlet 6, forinstance in the form of a chimney, but through a branched pipe a portionof the gases is recirculated to the upper part of the furnace 1 or tothe outlet 2 of the furnace. The means for this recirculation is notshown in FIG. 1 but the flow of gas is indicated by arrows, which aredrawn in full lines to illustrate the gas flow from the furnace l to thecyclone evaporator 3, and in dashed lines to illustrate the gas flowfrom the cyclone evaporator 3 to the outlet 6, part of which is returnedto the outlet of the furnace l (or the upper part of the furnace).

The cyclone evaporator 3 effects direct evaporation of thin liquorintroduced into it, by means of the hot combustion gases and steam fromthe liquor combustion furnace 1. This gas is caused to passcountercurrently to the thin liquor in a known manner. The evaporatedliquor is transferred from the cyclone evaporator 3 to the furnace l forcombustion therein as will be described below. The gases leaving throughthe outlet 6 still contain a substantial amount of heat the heat contentof which may be utilized in the pulp mill in a per se known manner, forinstance for heat production or pre-evaporation.

The liquor combustion furnace l which, as has already been mentioned, isa brick furnace having a circular cross-section and a verticallongitudinal axis, is shown in detail in F168. 2 and 3. In theembodiment shown the furnace is internally of circular-cylindrical formbut internally it may also be slightly circularconical, in which casethe diameter is to increase in the downward direction so that the bottomarea is larger than the top area.

The furnace 1 has a bottom 10 with an inclined flat bottom surface.Provided at the lowest point of the furnace bottom 10 is an outlet 12 inthe furnace wall so that the smelt soda formed through the liquorcombustion can discharge from the furnace through this outlet. Thefurnace has a wall 13 built of hard-burnt brick and is lined with anapproximately 25 cm thick layer 14 of firebrick which is highlyresistant to infiltration of alkali under the prevailing combustionconditions. The bottom section of the furnace is lined both in thebottom plane proper It] at 11 and up to a level of about 2 m above thebottom plane at 14' and also in an inner layer up to a level of about 1m at I4" by means of alkali resistant, refractory brick, for instance ofchrome brick type. The layer 11 of alkali resistant, refractory brick inthe bottom plane 10 rests on a layer 10' of stamping mass or porablecompound which is applied on the brick layer 10''. The stamping mass orporable compound may be of the same quality as the brick layer and isintended to prevent soda from penetrating down into the brickwork.

' The furnace has at its top a vault 15 of refractory brick ofsubstantially the same quality as the upper part of the furnace wall.This vault 15 rests on a collecting channel I6 for combustion wastegases built of brick and extending around the cylindrical outer wall ofthe furnace. Also the collecting channel 16 may be built insubstantially the same way as the furnace wall l3, 14. The vault 15rests freely relative to the cylindrical furnace wall 14 on thecollecting channel 16 so that the furnace wall 14 can move withouthaving any influence on the vault 15. The collecting channel 16 forwaste gases communicates with the furnace 1 through a gap between thevault l5 and the cylindrical furnace wall 14 and is provided withopenings 17, 18 covered by doors (not shown) for inspection and cleaningpurposes.

To make the furnace operate properly it is most important that the gasflow in the furnace shaft is as uniform as possible and for this reasonthe upper edge 19 of the furnace is inclined stepwise, as shown, so thatthe gap between the furnace wall 14 and the vault tapers successivelytowards a point (at 20) where the furnace outlet 2 (see also FIG. 1)adjoins the collecting channel 16.

The pipe 7 for evaporated liquor from the cyclone evaporator extendsinto the liquor combustion furnace 1 through the center of the vault 15(see also FIG. 1) and ends inside the furnace in a sprayer 21 locatedimmediately below the upper edge of the inner furnace wall 14 at a pointon the vertical central axis of the furnace. The sprayer is so designedand adjusted that the liquor is evenly distributed practicallythroughout the cross-section of the furnace but without coming intocontact with the furnace wall until it comes down at the bottom orbottom section of the furnace, as indicated by the cone of dispersion 22in FIG. 2. The droplet size is adjusted so that the liquor is evaporatedto dryness, as the droplets fall onto the furnace bottom, to such adegree that the desired combustion reaction in the bottom section 6 isobtained. On the other hand, the droplet size should not be too small sothat the liquor droplets would accompany the waste gases leaving thefurnace. A droplet size-of between l and 2 mm has been found suitablefor e.g., NSSC liquor which has a dry solids content of about 50 percentorganic material and about 50 percent sodium salts.

Air for the combustion is supplied to the furnace, on one hand, asprimary air in the lower section and, on the other hand, as secondaryair in one or more (two in the case shown) sections higher up in thefurnace. The primary air as well as the secondary air is supplied to thefurnace through nozzles which are arranged in groups in annular rowswith the inlets of the nozzles of each such row located in the sameplane and evenly distributed around the furnace. In FIG. 2, the primaryair nozzles are designated by 23 and the two rows of secondary airnozzles by 24 and 24' respectively. The row of primary nozzles maycomprise a large number of nozzles, say, about twenty, as indicated bycrosses. These may extend radially while the secondary air nozzles 24,24' may be tangentially directed to produce turbulence in the gas flowin the furnace. The fact that many primary nozzles 23 are used aroundthe furnace gives uniform reducing conditions in the lower part of thefurnace.

In its lower part the furnace has one or more oil burners 25 which maybe operated to start or, if desired, to sustain liquor combustion. Inthe lower part the furnace also has an inlet port 26 and, in-the part ofthe furnace wall 14 situated inside the collecting channel 16, a row ofapertures 27 for recirculating liquor or smelt to the furnace from thechannel 16.

The minimum height of the furnace depends substantially on the timerequired for dry evaporation of the falling droplets and on the timerequired for total combustion of the fume gases formed in the bottomsection of the furnace, where their path is indicated by arrows in FIG.2. A furnace height of about 9 m is suitable for combustion of wasteliquor from a NSSC mill. In that case, the primary air inlets 23 shouldbe placed about 065m above the furnace bottom, and the two secondary airinlets about 2.6 and 3.6 m, respectively, above said furnace bottomlevel.

The cross-section of the furnace is dependent on the heat load on thebottom which usually should not exceed about 1,200 Mcal per m and n.

The air combustion temperature is preferably adjusted so as notessentially to exceed about l,O00l,l00C, which temperature can beobtained in the shown furnace by combustion of waste liquor having aheat value of about 1,300 eff. kcal/kg liquor. The heat value of wasteliquors, evaporated to about 50 percent by weight, from NSSC pulpproduction is generally about l,200-l,400 eff. kcal/kg, half of the drysolids content being organic substance. If the waste liquor has a highercontent of organic material in the dry substance, the liquorconcentration must be done correspondingly lower. If the waste liquorshave a lower heat value, i.e., a lower content of organic material inthe dry substance, then the temperature required for the combustion ofthe liquor and the preparation of the smelt may be maintained byinjection and burning of oil by means of the oil burners 25 placed inthe furnace wall. The combustion temperature may be moderated as desiredby controlled returning of a certain amount of the waste gas from thecyclone evaporator 3 to the inlet 28 in the outlet 2.

The ratio of primary air to secondary air depends on the kind of liquorto be burned. Thus, a pure soda waste liquor may be burned withessentially all air as primary air. However, in burning NSSC liquor, theconditions in the lower zone of the furnace should be reducing, ifsulphur occurring therein is to be present as sulphide in the smelt. Inthat case, about 40 percent of the air is supplied as primary air. Inthis connection, however, there is always a risk of hydrogen sulphide(11 8) being present in the combustion gases. To counteract this, anexcess of air should be used, for instance so that the waste gasescontain about 5 percent 0,.

The degree of reduction in the combustion of sulphur-containing wasteliquors should be high and lie about percent. The thiosulphate contentis low, about 0.5 percent in the smelt, like the sulphate content whichis about 3 percent. As it is impossible completely to prevent someliquid droplets and ash from accompanying the waste gases out of thefurnace, a suitable direct evaporator is used for the evaporation, whichin the embodiment shown in FIG. 1 is a cyclone evaporator 3, asmentioned, since cyclone evaporators have been found especially suitableto free as far as possible the effluent waste gases, which are saturatedwith or rich in steam, from liquid droplets, and of course the wastegases are at the same time freed from the major portion of entrained ashwhich is returned to the furnace. The evaporator 3 is fed with wasteliquor (thin liquor) of a concentration of, for instance, about 20-25percent. At a gas temperature of about l,050C in the furnace l, theliquor may be evaporated to a concentration suitable for combustion,which may be about 50 percent but depends on the kind of liquor. Byrecirculating to the outlet 2 of the furnace 1 a portion of the cooledwaste gases enriched with steam, the hot waste gases may be cooled to atemperature suitable for direct evaporation, e.g. 350C.

The effluent combustion gases enriched with steam, which may maintain atemperature of about 100C, still contain the major portion of the heatcontent of the waste gases. Also liquor and ash particles in wet andsolid form, respectively, occur in a smaller amount in the waste gasesand may under certain circumstances cause inconveniences in the form ofmist if they are released directly into the air. Under certainconditions, e.g. when burning sulphur-containing waste liquors, alsosmall amounts of as well S as H 5 may occur in the effluent waste gases.Therefore, before the waste gases are released into atmosphere or usedfor the production of hot water or for heating purposes, it is advisablefirst to freethe gases from entrained particles and possiblyaccompanying acid constituents, such as $0 and H 8, by Washing the gasesin a scrubber of known type with circulating wash liquid at the dewpoint, whereby condensation of steam out of the gases is avoided and anenriched wash liquid can be obtained. This liquid can be recirculated tothe liquor for evaporation or be used otherwise in the process. Thepurified gas is then cooled down in a scrubber by direct contact withwater, which may be done either by supplying fresh water to the scrubberand directly utilizing discharged hot water for various purposes, or byfeeding the scrubber with circulating water which is indirectly cooled.

In the combustion of waste liquors leaving smelt soda as a residueduring combustion, it has thus been made possible to obtain asatisfactory reliability in operation as a result of the improvedfurnace construction according to this invention, suitable refractorylining, control of heat load and effective heat value of included wasteliquor and especially the method of introducing the liquor into thefurnace.

Regarding thefurnace, it should be mentioned that brick constructionwalls certainly have been tried before in the pulp industry forcombustion of waste liquors but due to unsatisfactory durability andreliability in operation one has proceeded to employing the system ofcooled furnace walls which now is generally used. This invention,however, provides a vertical shaft furnace of brick construction whichoperates excellently without any cooling whatsoever of walls or bottomsection and it has been established on the basis of carefully checkedtest running conditions that this furnace can be used several yearsunder corresponding conditions without requiring any rebuilding. Thefurnace of this invention thus represents a great step forward insettling the problem of neutralizing waste liquors and recoveringincluded chemicals in small mills and especially semichemical pulpmills.

Of course certain modifications of the furnace according to theinvention can be made within the scope of the inventive idea. Among theobjects of possible modifications there may first be mentioned themeasurements stated and the location of the air nozzles. Anotherpossible modification contributing to the desired uniform gas flow inthe upper part of the furnace and out through the outlet 2 is to arrangethe collecting channel 16 eccentrically relative to the furnace so thatit tapers towards the outlet 2.

What I claim to secure by Letters Patent is:

1. In a furnace having a substantially circular cross section and avertical axis for the continuous combustion of waste liquors containingcombustible organic substances combined with alkali and having means forspraying said waste liquors into the top of the furnace, in the upperpart of which moisture is evaporated from the waste liquor toconcentrate the liquor and in the lower part of which the concentratedliquor is subject to combustion, the gaseous products of combustionrising in said furnace to effect the evaporation of moisture from thewaste liquors being sprayed thereinto, the improvement comprising acircular furnace wall means extending circumferentially around theoutside of the top of said furnace wall for forming therewith an annularcollecting channel, a vault means mounted over the top of the furnaceand supported circumferentially on the outer circumference of saidannular channel means, said vault means being spaced from the upper endof said furnace wall to provide an annular, tapered gap therebetween,and an outlet means communicating with said annular channel means forremoval therefrom of combustion products and evaporated moisture, theannular, tapered gap being widest at that point diametrically oppositesaid outlet means and being narrowest at the point adjacent said outletmeans, whereby the combustion products and evaporated moisture will flowupwardly in the furnace substantially uniformly, through the annulartapered gap into said annular channel means and finally through saidoutlet means.

2. In a furnace as claimed in claim 1 the further improvement in whichthe upper end of the furnace wall lies in a plane inclined at an angleto the vertical axis of the furnace, said plane being lower at the pointdiametrically opposite to said outlet means and said vault means lies ina substantially horizontal plane above the wall of the furnace therebyforming the annular, tapered gap.

3. In a furnace as claimed in claim 1 the further improvement in whichsaid furnace wall, said vault means and said annular channel means areformed at least in part of refractory brick.

4. In a furnace as claimed in claim 1 wherein the annular gap betweenthe top of said furnace wall and said vault means is sufficient at allpoints to permit free expansion of said furnace wall. I

1. IN A FURNACE HAVING A SUBSTANTIALLY CIRCULAR CROSS SECTION AND AVERTICAL AXIS FOR THE CONTINUOUS COMBUSTION OF WASTE LIQUORS CONTAININGCOMBUSTIBLE ORGANIC SUBSTANCES COMBINED WITH ALKALI AND HAVING MEANS FORSPRAYING SAID WASTE LIQUORS INTO THE TOP OF THE FURNACE, IN THE UPPERPART OF WHICH MOISTURE IS EVAPORATED FROM THE WASTE LIQUOR TOCONCENTRATE THE LIQUOR AND IN THE LOWER PART OF WHICH THE CONCENTRATEDLIQUOR IS SUBJECT TO COMBUSTION, THE GASEOUS PRODUCTS OF COMBUSTIONRISING IN SAID FURNACE TO EFFECT THE EVAPORATION OF MOISTURE FROM THEWASTE LIQUORS BEING SPRAYED THEREINTO, THE IMPROVEMENT COMPRISING ACIRCULAR FURNACE WALL MEANS EXTENDING CIRCUMFERENTIALLY AROUND THEOUTSIDE OF THE TOP OF SAID FURNACE WALL FOR FORMING THEREWITH ANANNULALR COLLECTING CHANNEL, A VAULT MEANS MOUNTED OVER THE TOP OF THEFURNACE AND SUPPORTED CIRCUMFERENTIALLY ON THE OUTER CIRCUMFERENCE OFSAID ANNULAR CHANNEL MEANS, SAID VAULT MEANS BEING SPACED FROM THE UPPEREND OF SAID FURNACE WALL TO PROVIDE AN ANNULAR, TAPERED GAPTHEREBETWEEN, AND AN OUTLET MEANS COMMUNICATING THE SAID ANNULAR CHANNELMEANS FOR REMOVAL THEREFROM OF COMBUSTION PORIDCTS AND EVAPORATEDMOISTURE, THE ANNULAR, TAPERED GAP BEING WIDEST AT THAT POINTDIAMETRICALLY OPPOSITE SAID OUTLET MEANS AND BEING NARROWEST AT THEPOINT ADJACENT SAID OUTLET MEANS, WHEREBY THE COMBUSTION PRODUCTS ANDEVAPORATED MOISTURE WILL FLOW UPWARDLY IN THE FURNACE SUBSTANTIALLYUNIFORMLY,THROUGHT THE ANNULAR TAPERED GAP INTO SAID ANNULAR CHANNELMEANS AND FINALLY THROUGH SAID OUTLET MEANS.
 2. In a furnace as claimedin claim 1 the further improvement in which the upper end of the furnacewall lies in a plane inclined at an angle to the vertical axis of thefurnace, said plane being lower at the point diametrically opposite tosaid outlet means and said vault means lies in a substantiallyhorizontal plane above the wall of the furnace thereby forming theannular, tapered gap.
 3. In a furnace as claimed in claim 1 the furtherimprovement in which said furnace wall, said vault means and saidannular channel means are formed at least in part of refractory brick.4. In a furnace as claimed in claim 1 wherein the annular gap betweenthe top of said furnace wall and said vault means is sufficient at allpoints to permit free expansion of said furnace wall.